1. Field of the Invention
The present invention relates to a metallic support catalyst, which is adapted for purifying exhaust gases emitted from internal combustion engines, and which comprises a metallic honeycomb body, and a catalyst ingredient loaded on the metallic honeycomb body.
2. Description of Related Art
As automotive exhaust gas emission control becomes strict, automobiles are equipped with an exhaust gas purifying apparatus. The following methods are available for purifying automotive exhaust gases; namely: a thermal reactor method, a lean-burn method, an engine-modification method, and a catalyst method. Many automobiles employ a catalyst method.
As for an automotive exhaust-gas-purifying catalyst, a Pt, Pd, or Rh-based catalyst is used. An automotive exhaust-gas-purifying catalyst can be classified into granular one and monolithic one in terms of shape. Monolithic catalyst is made from either ceramics or metal. Ceramics catalyst is made by the following processes; namely: ceramics is calcinated into a honeycomb-shaped support. The support is accommodated in a metallic cylinder. A .gamma.-Al.sub.2 O.sub.3 powder with a catalyst ingredient (e.g., Pt, or the like) loaded is deposited on the support. The ceramics monolithic support catalyst is susceptible to mechanical shocks, and exhibits large resistance against exhaust gases. Accordingly, metallic monolithic support catalysts have recently come to be used widely in order to reduce pressure-loss in exhaust system and enhance the heat resistance of support.
For instance, a metallic monolithic catalyst is made as follows; namely: an Fe--Cr--Al stainless steel plate is rolled into a flat foil in a thickness of a few dozens of micrometers. The Fe--Cr--Al stainless steel plate, for example, includes Cr in an amount of 20% by weight, Al in an amount of 5% by weight, and the balance of Fe and inevitable impurities. Part of the resulting flat foil is formed into a corrugated foil. The flat foil and the corrugated foil are laminated alternately, or formed as a roll so as to prepare a honeycomb body. The honeycomb body is accommodated in a metallic cylinder to make a substrate. The substrate is coated with .gamma.-Al.sub.2 O.sub.3 to form a catalyst ingredient loading layer. Then, a catalyst ingredient, such as Pt, Rh, and Pd, is deposited on the loading layer.
In the metallic monolithic catalyst or metallic support catalyst, it has been required to improve the purifying characteristic immediately after starting engine (i.e., warm-up characteristic). A variety of attempts have been made so far. For example, in order to upgrade the warm-up characteristic of metallic support catalyst, it is effective to reduce the heat capacity of metallic support. Accordingly, the cell density of honeycomb body has been reduced, or the foil, forming metallic-honeycomb body, has been perforated. However, such attempts have decreased the geometric surface area of metallic honeycomb support.
On the other hand, in order to enhance the conversion of metallic support catalyst, it is effective to increase the geometric surface area of metallic support. Accordingly, the cell density of metallic support has been increased, or the foil, forming the metallic honeycomb support, has been provided with projections or is corrugated. However, such attempts have increased the heat capacity of metallic honeycomb support, and resulted in impairing the warm-up characteristic of metallic support catalyst.
Metallic support catalysts are required to exhibit good warm-up characteristic as well as high conversion. It is understood from the foregoing description that improving the warm-up characteristic results in reducing the conversion, and that enhancing the conversion results in deteriorating the warm-up characteristic. Thus, it is difficult to satisfy both of these characteristics at the same time.